1. Field of Invention
The present invention relates to a brake stroke simulator and, more particularly, to a brake stroke simulator for achieving good stroke feeling in an electronic control brake apparatus for a vehicle.
2. Description of Related Art
A vehicular electronic brake apparatus having a brake stroke simulator is disclosed in, for example, Japanese Patent Application Laid-Open No. HEI 6-211124. In such a brake apparatus, the brake stroke simulator is connected in communication to a master cylinder that generates master cylinder pressure in accordance with the brake pedal depressing force.
This conventional brake apparatus has a high-pressure source that always generates a predetermined liquid pressure regardless of whether there is a brake pedal depressing force. At the time of depression of the brake pedal, the communication between the master cylinder and the wheel cylinders is shut down and the wheel cylinder pressure is regulated by using the high-pressure source as a liquid pressure source, as long as the brake system is operating normally. In this case, brake fluid from the master cylinder flows into the brake stroke simulator.
The brake stroke simulator provided in a brake apparatus as described above has a piston that is displaced by master cylinder pressure, and a plurality of disc springs that urge the piston in a direction opposite to the direction of the force caused by master cylinder pressure. The disc springs are superposed on top of each other. The disc springs produce a reaction force when the piston is displaced by master cylinder pressure. The displacement of the piston increases with increases in pedal stroke. The reaction force produced by the disc springs increases with increases in the piston displacement. Therefore, the reaction force by the disc springs increases with increases in pedal stroke.
The master cylinder pressure increases as the reaction force produced by the disc springs increases. A pedal reaction force that is transmitted to the brake pedal increases with increases in the master cylinder pressure. Therefore, the brake stroke simulator is able to increase the pedal reaction force as the depression of the brake pedal is increased.
In a normal manual brake apparatus (hereinafter, referred to as "normal brake apparatus"), the master cylinder pressure is directly supplied to the wheel cylinders. In this apparatus, the pedal reaction force is in a non-linear relation to the pedal stroke. More specifically, the pedal reaction force comparatively gently changes relative to changes in the pedal stroke if the pedal stroke is within a range of small strokes. If the pedal stroke is within a large stroke range, the pedal reaction force comparatively sharply changes with changes in the pedal stroke.
Such a non-linear characteristic of the pedal reaction force relative to the pedal stroke similar to that achieved in normal brake apparatuses can be achieved, by using a stack of spring discs as in the brake stroke simulator described above. Therefore, while the communication between the master cylinder and the wheel cylinders in the related-art brake apparatus is shut down, the related-art brake apparatus is able to provide pedal operating feeling similar to that provided by normal brake apparatuses.
However, the characteristic of the related-art brake stroke simulator varies as the superposed condition of the disc springs changes. It is difficult to always maintain a constant superposed condition of the disc springs in the production process of brake stroke simulators. Consequently, the conventional brake stroke simulators are subject to variations among individual simulators.